It is well known that an antenna is a device for mutual transformation between guided waves in transmission wires and electromagnetic waves in space, and serves as an essential component in a wireless unit.
In a case that there is not any determined positional (azimuth, distance, height) relationship between wireless communication devices, an ideal radiation pattern is generally required to be omni-directional in the horizontal plane and be similar to a half-wave dipole in the vertical plane.
As the wireless communication technology develops, more and more frequency bands are required to be supported by antennas, which brings significant challenges in antenna design. According to the antenna theory, an effective length of a radiator should be comparable to ¼ wavelength to achieve higher efficiency. Comprehensively considering of multiple aspects including volume, weight, cost, portability, aesthetic measure and the like, as an ideal external omni-directional antenna for a wireless communication device, its length-diameter ratio (or length-width ratio) should be greater than 6, and a structure of the antenna is preferable to be axisymmetric to maintain a good roundness for the radiation pattern in the horizontal plane. If a planar structure is adopted, a maximum width of the radiator should be less than ⅛ of a wavelength of the highest operating frequency, so as to maintain a non-roundness of the radiation pattern in the horizontal plane within 3 dB.
It is with superior performance for a coaxial half-wave dipole antenna with excellent roundness in the horizontal plane as well as high radiation efficiency under good impedance matching. However, a standard coaxial half-wave dipole antenna is of a single frequency, and has a bandwidth typically no more than 10% when its Voltage Standing Wave Ratio (VSWR) is less than 2. Existing methods for spreading a bandwidth of an external antenna include:                1. Disposing multiple radiating elements, so that there will be multiple resonance paths for current;        2. Disposing a parasitic radiating element so as to spread the bandwidth utilizing coupling between a main radiating element and the parasitic radiating element.        3. Thickening and widening the radiating element;        4. Disposing a sleeve over the antenna to enable the antenna to resonate in a high frequency band.        
However, the methods mentioned above are limited in being applied to a small-sized antenna. For example, they are severely restricted by a radial/lateral size of the antenna. Specifically, in the first method, there are certain distances between radiating elements respectively, thereby being not covering full frequency bands due to being limited by space to not provide enough radiating elements. The second and fourth methods can spread a single band antenna to a dual-band or multi-band antenna, but a bandwidth for each frequency band will be not wide enough and a bandwidth of the VSWR for each frequency band is not ideal, and furthermore, distances are non-optimal from main radiating elements to parasitic radiating element and the sleeve. The third method requires to largely thicken the radiating element, which makes it difficult to be applied to small-sized antennas.